High-temperature ovens, called reactors, are used to create structures of very fine dimensions, such as integrated circuits on semiconductor substrates. One or more substrates, such as silicon wafers, are placed on a wafer support inside the reaction chamber. Both the wafer and support are heated to a desired temperature. In a typical wafer treatment step, reactant gases are passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure. Through subsequent processes, these layers are made into integrated circuits, with a single layer producing from tens to thousands or even millions of integrated devices, depending on the size of the wafer and the complexity of the circuits.
Various process parameters must be carefully controlled to ensure the high quality of the resulting layers. One such critical parameter is the temperature of the wafer during each treatment step of the processing. During CVD, for example, the deposition gases react at particular temperatures and deposit on the wafer. If the temperature varies across the surface of the wafer, uneven deposition of the reactant gas occurs. Accordingly, it is important that wafer temperature be stable and uniform at the desired temperature before the treatment begins.
Similarly, non-uniformity or instability of temperatures across a wafer during other thermal treatments can affect the uniformity of resulting structures. Other processes for which temperature control can be critical include oxidation, nitridation, dopant diffusion, sputter depositions, photolithography, dry etching, plasma processes, and high temperature anneals.
In certain batch processors (i.e., reactors which process more than one wafer at a time), a plurality of wafers are placed on a relatively large-mass susceptor made of graphite or other heat-absorbing material to help the temperature of the wafers remain uniform. In this context, a "large-mass" susceptor is one which has a large thermal mass relative to the wafer. The thermal mass of a solid, or its lumped thermal capacitance, is given by the equation: EQU C.sub.T =.rho.Vc
where:
.rho.=the density of the solid, PA1 V=the volume of the solid, and PA1 c=the specific heat (heat capacity) of the solid.
Thus, the thermal mass is directly related to its mass, which is equal to the density times volume, and to its specific heat.
One example of a large-mass susceptor is shown in U.S. Pat. No. 4,496,609 issued to McNeilly, which discloses a CVD process wherein the wafers are placed directly on a relatively large slab-like susceptor and maintained in intimate contact to permit a transfer of heat therebetween. The graphite susceptor supposedly acts as a heat "flywheel" which transfers heat to the wafer to maintain its temperature uniform. The goal is to reduce transient temperature variations around the wafer that would occur without the "flywheel" effect of the susceptor. Despite use of a large-mass susceptor, however, maintenance of uniform conditions across several wafers in a batch process remained difficult.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons, including the greater precision with which process parameters can be monitored and controlled, as compared to batch-processing. Typical wafers are made of silicon with one common size having a diameter of 200 mm and a thickness of 0.725 mm, though smaller wafers (e.g., 100 mm, 125 mm, 150 mm) have also been used. Recently, larger silicon wafers having a diameter of 300 mm and a thickness of 0.775 mm have been proposed, as they even more efficiently exploit the benefits of larger single-wafer processing. Even larger wafers are contemplated for the future.
An example of a single-wafer reactor is shown in U.S. Pat. No. 4,821,674, which includes a circular rotatable susceptor having a diameter slightly larger than the wafer. Though such a susceptor has a lower thermal mass than the aforementioned slab-type batch processing susceptor, the thermal mass of the susceptor remains large compared to the thermal mass of the wafer.
One way in which process control is improved with single-wafer processing is by the ability to measure and control the temperature at various positions about the single wafer, which is impractical to perform for each of a batch of wafers. A plurality of temperature sensors, such as thermocouples or pyrometers, measure the temperature at various points surrounding the wafer. For example, one thermocouple can be placed near the leading edge of the wafer (the edge closest to the inlet for reactant gases), one near the trailing edge, one at a side, and one directly below the wafer. Temperature data from the thermocouples is sent to a temperature controller, which analyzes the data and adjusts the power output of a plurality of heat sources to keep the temperature at these various points uniform and at the desired level. The heat sources are typically radiant heating elements, or lamps, which have the advantage of rapid response to the controller.
Improved process control by single-wafer processing, however, comes at the expense of sharply reducing the number of wafers which can be processed in a given length of time (i.e., process throughput), as compared to batch processing. Since wafers can be processed only one at a time, any reduction of process time will significantly improve wafer throughput for single-wafer reactors.
One factor which critically affects throughput is the speed with which the wafer temperature can be ramped. Such temperature ramping can be required at several points in a given process. For example, a cold wafer must be heated to the appropriate treatment temperature. The process itself may also require different temperatures for different treatment steps. At the end of a process, the wafer ordinarily must be cooled to a level which the wafer handling device can tolerate.
More recently, it has been suggested that processing times can be reduced by using wafer holding fixtures of lower thermal mass than that of conventional susceptors. U.S. Pat. No. 4,978,567 describes such a low mass wafer holding fixture. The lower thermal mass of the wafer holder/wafer combination facilitates rapid heating and cooling of the wafer for Rapid Thermal Processing (RTP) systems. Throughput can also be increased in connection with other processes involving heating or cooling of a substrate to be processed.
Despite advancements in the design of single-wafer reactors and low mass wafer holding fixtures, heating and cooling steps continue to represent a significant percentage of processing time and a limitation on the attainable level of throughput. Consequently, there remains a need for reduced temperature ramp times.